It’s all in the family! But how? The biology of inheritance Part 2

Author: Shweta Ramdas

Editors: Molly Kozminsky, Christina Vallianatos, Bryan Moyers

If you haven’t been living under a rock for the last five years, you have definitely come across headlines to the tune of “Researchers Find Gene for X”, where X can be anything from happiness, to political affiliation, to your preference for cilantro. There are quite a few people who respond to these studies with “but surely that’s not genetic!” I work on the genetics of psychiatric disorders and have fielded this question from most people with whom I discuss my research: “Isn’t something like depression just caused by things that happen to you or your upbringing? Why do we place the blame on genetics instead?”

Why study the genetic inheritance of a trait if it isn’t even passed on by biological mechanisms? For instance, members of a family often support the same sports teams, but surely that’s no reason to launch a new study analyzing the genetic basis of support for Manchester United. Fortunately, geneticists have thought about this for a long time. To answer this question, the first step in a genetic study is measuring something called heritability, which is how frequently a trait is passed from parents to child through genetics.

The first attempts to identify heritability looked at whether the trait ran in families more often than in the general population. For instance, one in ten people in the world have diabetes, but within families with diabetes we often find half of all family members are affected. Members of a family share more DNA than two random people in a population; for instance, you share half your DNA with your mother and with your sibling. We see that one family member is more likely to have a disease if a close relative has it (called concordance), and therefore we can infer that this increased sharing of disease is because of shared genetic material.

But is this necessarily true? Families share things apart from genetic material: homes, microbes, diet – in short, the environment. If members of a family show increased sharing of a disease like diabetes, then it could be because of any of these, for instance, because of the family’s habit of dessert after every meal. Enter: twin studies. Twin studies look at shared disease incidences between twins, specifically whether identical twins (sharing both genes and the environment) show greater concordance than fraternal twins (sharing fewer genes, but equal environment.) It was Francis Galton, Darwin’s cousin, who first used twin studies to determine if England’s “men of genius” were a product of nurture or breeding.

The next stage in the history of twin studies followed an unsavory path: it was a Nazi geneticist Josef Mengele who brought them to our attention. Twin pairs were called out in Nazi camps and experimented upon to identify genetic influences for traits like eye color, and traits like depression and homosexuality, that were “undesirable” in the general population. Mengele wanted to establish a theory of heredity that would prove the Nazi doctrine of Aryan superiority.

Adoption studies take a different philosophy to measuring heritability. If we compare identical twins who have been separated at birth, we can get a near-perfect separation of genes and the environment. These twins have near-identical genetic material but very different environments – can thus be used to estimate heritability. In 1968, a research group found that children of parents with schizophrenia were just as likely to be affected by the disease regardless of being raised by biological or adoptive parents. This went a long way in establishing that the disorder had a genetic component.

These studies of estimating “heritability” are not completely accepted as gospel in the world of genetics. For instance, twin/sibling studies assume that the environment within a family is the same for both twins, whether they are identical or fraternal. However, if identical twins are treated more similarly by parents than fraternal twins, then the environmental similarity is greater for identical twins than for fraternal twins. This might make geneticists overemphasize the influence of genetics because identical twins have more similar genes and environment than fraternal twins do. On the other hand, if identical twins share a less similar environment than fraternal twins (from extra efforts to give them their own identities) then heritability is under-estimated. Thus we are still coming up with better measures of heritability, though it is generally accepted that current measures are reasonable, but not perfect in estimating biological inheritance.

Biology certainly plays a large role in the continuity of a species, but much of human society is founded upon cultural means of transmission. Certainly there is no “handshake as a form of greeting” gene or a “Mandarin language” gene. We have ways to approximate differences between biological and non-biological ways of inheritance that help us understand that there are biological underpinnings to behavioral traits like depression. Such traits certainly aren’t as inheritable as they are made out to be, but they sure are heritable, and the hunt to understand their biological basis is an ongoing one.

In parallel are the ethical issues of how much we can attribute genetics to issues of bad behavior. It’s important for biologists to determine how much these traits are influenced by our genetics because it’s influencing societal decisions. For instance, some court cases are using genetic markers as mitigating factors in sentencing. Whether or not this is appropriate needs to be informed by both science and ethical discussion.

In Part 3 of this series, I’ll talk about the ethical issues that have risen as we continue to understand the role of genetics in behavior. Just how much leeway should someone be given because of a genetic vulnerability? Watch this space to read more.

About the author

Shweta is a graduate student in Bioinformatics at the University of Michigan. Her research involves computational methods to understand the genetic basis of psychiatric disease. Her undergraduate degree is from the National University of Singapore where she studied computational biology. Outside of research, Shweta enjoys reading, yoga, and figuring out the genetic basis for being a muggle. Follow Shweta on Twitter.